The invention concerns an optical spectrometer, in particular a Fourier transform (FT) spectrometer, with an interferometer having a means for varying the optical path difference comprising a drive, and with a detector for recording optical signals from the interferometer and converting them into electrical analog signals, wherein data acquisition electronics are connected to the detector, which comprise at least one analog-to-digital converter (ADC), digitizing the electrical analog signals in a time-equidistant manner, and, if necessary, further signal processing elements.
A spectrometer of this type is known from the article entitled "New approach to high-precision Fourier transform spectrometer design" by J. W. Brault in the journal Appl. Optics, Vol. 35, No. 16, pages 2891-2896, Jun. 1, 1996.
Optical spectroscopy, in particular infrared Fourier transform (IR-FT) spectroscopy is one of the most effective tools available to the analytical chemist in research, application and process control. Common methods of recording such optical spectra are described in all details e.g. in a series of articles by J. Gronholz and W. Herres with the title "Datenverarbeitung in der FT-IR Spektronskopie" (data processing in FT-IR spectroscopy) in the journal Comp.Anw.Lab., Edition 5/1984, pages 352-356, Edition 6/1984, pages 418-425 and Edition 5/1985, pages 230-240. In this connection, zero crossings of a reference interferogram, which is recorded e.g. by an HeNe laser, are measured in their temporal sequence and the simultaneously recorded effective interferogram is digitized at these zeroes.
The hitherto common method, applied in optical FT spectrometers from the infrared to the ultraviolet range, of sampling the detector signal in a spatially equidistant manner with reference to the position of the interferometer mirror (in a Michelson interferometer), however, does not allow exact compensation of the amplitude and phase responses of the detector, since the actual speed of the interferometer mirror is not known. For this reason, it is not possible to compensate differences and distortions of the transit time between effective signal and reference signal due to an apparatus function. Any speed variation of the mirror drive will therefore cause side-band modulation in the spectral lines. For this reason, operational methods for spectrometers of this type are not suited for systems with heavy mechanical disturbances, e.g. in the vicinity of vibration generating machines or for spectrometers which are mounted e.g. on movable vehicles.
The initially cited publication by J. W. Brault describes, in contrast thereto, a method of compensating the amplitude and phase responses which can be used with particular efficiency in a system with time-equidistant sampling of the optical signal. By means of "pre" digitization of the effective signal by means of the ADC with fixed time periods, the apparatus function of the detector and the further signal processing elements can essentially be removed from the spectra by deconvolution.
The time-equidistant sampling enables determination of the development with time of the detector signal. A digital filter (called compensation filter) can be applied to said sampled signal, which filter comprises the reciprocal complex frequency response (consisting of amplitude and phase responses) of the detector and the further signal processing elements. At the output of this filter, values are obtained which correspond to the optical signal at the input of the detector multiplied by its spectral responsivity delayed merely by a constant time period. The signal is independent of the driving speed of the interferometer mirror.
In order to obtain from the detector signal an interferogram which is independent of the speed variations of the movable mirror in the interferometer, spatially equidistant sampling of the IR detector signal is necessary. In order to convert the signal, which was sampled at equal time intervals, into a spatially equidistantly sampled signal, signal values are calculated by means of a digital filter with constant delay (called an interpolation filter) at those points in time, at which the optical path difference in the interferometer achieves certain values, i.e. values, at which the movable mirror/s in the interferometer is/are at certain locations. Since the compensation filter and the interpolation filter operate in the time domain in each case, they can be applied one after the other.
Since the two filters are applied one after the other and both are time domain filters, they can be combined in one filter by convolving their filter coefficients. This reduces the requirements concerning the storage need and the speed of the digital filter processor.
Finally, a further digital filter (called spatial frequency filter) can be applied to the resulting values of the combined compensation and interpolation filter, which carries out reduction of the data to the desired spectral range. This filter cannot be linked with the combined filter since it has to be applied to spatially equidistant sampling values; however, it can be carried out by the same processor.
The time-equidistant sampling method allows simultaneous recording of the variations in time and the absolute positions of the zero crossings of the reference signal. After corresponding conversion of the ADC signals to spatially equidistant positions of the mirror (interpolation filter), a signal quality can be achieved which is at least equal to the one of the above-described spatially equidistant sampling method, wherein, however, the experimenter is given considerably more flexibility since not only the zero crossings but any intermediate values can be used for digitization. A further, very important advantage of the method according to Brault consists in the possible correction of the apparatus function by the above-described compensation filter which corresponds to deconvolution of the transfer function of the detector from the spectra.
In known spectrometers, the detector signal of which is not sampled in a time-equidistant manner but in a spatially equidistant manner, the expense connected with the apparatus is considerably higher. The sampling signal does not have a constant frequency but depends on the speed of the drive. For this reason, it is not possible to synchronize a switched power supply by means of the sampling signal. In order to achieve the required measurement accuracy, which should be larger than 16 bit, the product of ripple and interference suppression of the supply voltage of the data acquisition electronics has to be less than 1/(2.sup.16) of the supply voltage range. It is not possible to achieve such accuracy with the currently known switched power supplies. For this reason, in spectrometers with spatially equidistant signal sampling, the data recording part has to be supplied from a separate linear power supply and furthermore, has to be galvanically separated from the switched supply of the electronic control unit of the drive. The required linear power supply, the use of separate voltage supplies and the galvanic separation of the data acquisition electronics and the electronic control unit of the linear drive, however, cause high expenses.
For this reason, it is an object of the present invention to provide an optical spectrometer comprising the initially described features, on which a method according to the Brault proposition can be carried out, wherein, in contrast to known spectrometers with spatially equidistant sampling of the detector signal, the design of the spectrometer is considerably cheaper and more compact with respect to its voltage supply means, and wherein galvanic separation of the data acquisition electronics and the electronic control unit of the drive is not required.